The present invention relates to a ferromagnetic sputtering target used for depositing a thin film in a magnetic recording medium, in particular depositing a granular film on a magnetic recording medium for a hard disk in which the perpendicular magnetic recording system is used. The present invention also relates to a nonmagnetic grain-dispersed ferromagnetic sputtering target capable of suppressing discharge abnormalities of oxides that are the cause of particle generation during sputtering and a producing method thereof.
Although a wide variety of systems are available for a sputtering device, a magnetron sputtering device equipped with a DC power supply is widely used in view of high productivity to deposit the above magnetic recording films. The sputtering method refers to one in which a substrate serving as a positive electrode is placed facing a target which serves as a negative electrode, and high voltage is applied between the substrate and the target under an inert gas atmosphere to generate an electric field.
At this time, the inert gas is ionized to form plasma which comprises electrons and cations. When the cations in the plasma hit the surface of the target (the negative electrode), atoms which constitute the target are ejected. A film is formed when the ejected atoms adhere to the surface of the opposing substrate. The method employs the principle that a material which constitutes the target is deposited on the substrate by a series of these operations.
Meanwhile, referring to the development of magnetic materials, in the field of magnetic recording represented by hard disk drives, materials based on ferromagnetic metals such as Co, Fe or Ni are used as materials for magnetic thin films which serve for recording. For example, in recording layers of hard disks in which the longitudinal magnetic recording system is used, Co—Cr based or Co—Cr—Pt based ferromagnetic alloys having Co as a main component are used.
In addition, for recording layers of hard disks employing the perpendicular magnetic recording system which has been recently put in practical use, composite materials comprising a Co—Cr—Pt based ferromagnetic alloy having Co as a main component and a nonmagnetic inorganic substance are often used.
In many cases, a magnetic thin film in a magnetic recording medium such as a hard disk is produced by sputtering a ferromagnetic material sputtering target having the above materials as main components because of high productivity.
Methods of producing the foregoing ferromagnetic sputtering target include the dissolution method and the powder metallurgy method. Which methods may be used depends on required properties. But a sputtering target comprising a ferromagnetic alloy and grains of a nonmagnetic inorganic substance, which is used for recording layers for hard disks of the perpendicular magnetic recording system, is generally produced by the powder metallurgy method. This is because producing is difficult by the dissolution method since the grains of an inorganic substance need to be uniformly dispersed in the alloy base.
For example, a method of obtain a sputtering target for a magnetic recording medium is proposed, the method comprising: performing mechanical alloying on an alloy powder having a metal phase produced by the rapid solidification method and a powder for a ceramics phase to uniformly disperse the powder for a ceramics phase in the alloy powder; and performing molding by hot press (Patent Literature 1). In this case, the structure of the target shows an appearance where the base materials are connected in a form of soft roe (cod sperm), which are surrounded by SiO2 (ceramics) (FIG. 2 of Patent Literature 1) or an appearance where the base materials are dispersed in a form of a thin string (FIG. 3 of Patent Literature 1). Other figures are assumed to show the similar structures although they are not clear. These structures have problems described below, and cannot be considered as a suitable sputtering target for a magnetic recording medium. Note that spherical materials shown in FIG. 4 of Patent Literature 1 are of powder, and are not the structure of the target.
Further, even in a case where an alloy powder produced by the rapid solidification method is not used, a ferromagnetic sputtering target can be produced by preparing commercially available raw powders for each component which will constitute the target, weighing out the raw powders to obtain a desired composition, mixing by a known method such as ball milling, and molding and sintering the mixed powder by hot press.
For example, a method of obtaining a sputtering target for a magnetic recording medium is proposed, the method comprising: mixing a mixed powder obtained by mixing a Co powder, a Cr powder, a TiO2 powder and a SiO2 powder, with a Co spherical powder by a planetary mixer, molding the resulting powder mixture by hot press.
In this case, the structure of the target shows an appearance where there is a spherical phase (B) in a phase (A) in which grains of an inorganic substance are uniformly dispersed in the metal base (FIG. 1 of Patent Literature 2). The structure as described above is suitable in terms of improving magnetic leakage flux, while in terms of suppressing particle generation during sputtering, it can hardly be considered as a suitable sputtering target for a magnetic recording medium.
In addition, a method of obtaining a sputtering target for forming a magnetic recording medium thin film is proposed, wherein the method comprising: mixing a Co—Cr binary alloy powder, a Pt powder and a SiO2 powder, and hot pressing the resulting mixed powder (Patent Literature 3).
In the structure of the target in this case, a Pt phase, a SiO2 phase and a Co—Cr binary alloy phase were observed, and a dispersed layer was observed around a Co—Cr binary alloy layer although not shown in the figures. Such a structure as described above, it can hardly be considered as a suitable sputtering target for a magnetic recording medium.
Further to these described above, there are a few more proposals for developing a magnetic material. For example, a perpendicular magnetic recording medium having SiC and SiOx (x: 1 to 2) is proposed in Patent Literature 4. Patent Literature 5 describes a magnetic material target containing Co, Pt, a first metal oxide, a second metal oxide and a third metal oxide.
Further, in Patent Literature 6, proposed is a sputtering target comprising a Co—Pt matrix phase and a metal oxide phase having an average grain diameter of 0.05 μm or more and less than 7.0 μm. Also proposed is that controlling growth of crystal grains to obtain a target having low magnetic permeability and high density, thereby increasing deposition efficiency.
Furthermore, Patent Literature 7 describes a nonmagnetic grain-dispersed ferromagnetic sputtering target in which a shape of a nonmagnetic material (smaller than a virtual circle having a radius of 2 μm) is defined with a material having ferromagnetic materials Co and Fe as main components and having a material selected from the group consisting of oxide, nitride, carbide and silicide.
Moreover, Patent Literature 8 describes a nonmagnetic grain-dispersed ferromagnetic sputtering target in which nonmagnetic material grains comprising, oxides smaller than a virtual circle having a radius of 1 μm are dispersed in a Co—Cr alloy ferromagnetic material. It also describes a sputtering target in which the grain-diameter thereof is defined in detail. In addition, Patent Literature 9 describes a magnetic film having a granular structure.
As described above, for nonmagnetic grain-dispersed ferromagnetic sputtering targets such as a Co—Cr—Pt-oxide, use of SiO2, Cr2O3 and TiO2 as an oxide has been proposed, and defining a shape of the oxide has been further proposed. However, these oxides can cause abnormal electric discharge since they are insulators. Therefore, this abnormal electric discharge will pose a problem of particle generation during sputtering.
The amount of levitation of a magnetic recording head is becoming smaller every year as the recording density of HDD is improved. Therefore, the size and the number of particles permissible on magnetic recording medium are becoming even stricter. It is known that many of particles generated during the deposition of a granular film are oxides from the target. As one method of suppressing such particle generation, finely dispersing oxides in a target into a parent phase alloy appears to be effective.
In addition to Patent Literatures 6 to 8 described above, a smaller grain size of a metal oxide is also proposed in Patent Literatures 10 to 19 described below. That is, Patent Literature 10 describes that an average grain diameter of grains formed in a metal oxide phase is 0.05 μm or more and less than 7.0 μm.
Patent Literature 11 describes that the grain diameter along the long axis of a ceramics phase is 10 μm or less. Patent Literature 12 describes that an oxygen-containing substance or an oxide phase is 50 μm or less. Patent Literature 13 describes that an average grain diameter of grains formed in an oxide phase is 3 μm or less. Patent Literature 14 describes that silica grains or titania grains satisfy 2≦Dp/Dn at a cross section perpendicular to the main surface of a sputtering target where Dn represents a grain diameter in the direction perpendicular to the main surface of the sputtering target, and Dp represents a grain diameter in the direction parallel to the above main surface. Patent Literature 15 describes that there are 500 chromium oxide aggregates present per mm2 
Moreover, Patent Literature 16 describes that in a Co based alloy sputtering target containing silica, Cr or Pt, wherein the silica phase is in the range 0.5 to 5 μm and a hydrophobic silica powder can be used for manufacture. Patent Literature 17 describes a sputtering target used for producing a magnetic recording medium, wherein the grain diameters of oxides are 10 μm or less. Patent Literature 18 describes a Co—Cr—Pt—C based sputtering target, wherein an average crystal grain size in the matrix is 50 μm or less, and carbide is dispersed in the structure. Patent Literature 19 describes a magnetic recording medium wherein crystal grains constituting a magnetic thin film are separated from a crystal grain boundary portion comprising a non-ferromagnetic non-metal phase. However, none of these grain-micronizing conditions are sufficient, and the demand for further improvement still remains at the present.